How to check if a string contain valid hash - ruby-on-rails

Question

i encounter a problem when i want to validate the input string if it contain a valid hash before execute eval on it, for example:
"{:key=>true}" if i run eval it return a correct hash, but if i run eval on this string "{:key=>true" i get syntax error of expected token:
(eval):1: syntax error, unexpected end-of-input, expecting '}' {:redirect=>true ^
i did tried some basic validation but no luck so far.
so basically what i want to know is how to validate a that a string contain correct hash format.

Answer 1

You can't tell without actually parsing it as Ruby, and (assuming you trust the string), the simplest way of parsing it is to simply do the eval call, and handle the exception:
begin
hash = eval(string)
rescue SyntaxError
# It's not valid
end
This is precisely what exceptions are for, instead of littering your code with checks for whether operations will succeed, you just perform the operations, and handle errors that occur.

Answer 2

To validate the string you can use Kernel#eval + checking the type:
def valid_hash?(string)
eval(string).is_a?(Hash)
rescue SyntaxError
false
end
Usage:
string = "{:key=>true}"
valid_hash?(string)
#=> true
string = "I am not hash"
valid_hash?(string)
#=> false

Answer 3

I had a similar problem, but I don't like the eval solution, because it's not safe.
I used the JSON gem and modified the string to match the JSON syntax.
Assuming you only have symbol keys.
'key: value' to '"key": value'
':key => value' to '"key": value'
string1 = "{:key_1=>true,key_2:false}"
string2 = "{:key=>true}"
string3 = "no hash"
def valid_hash?(string)
begin
string = string.gsub(/(\w+):\s*([^},])/, '"\1":\2')
#=> "{:key_1=>true,\"key_2\":false}"
string = string.gsub(/:(\w+)\s*=>/, '"\1":')
#=> "{\"key_1\":true,\"key_2\":false}"
my_hash = JSON.parse(string, {symbolize_names: true})
#=> {:key_1=>true, :key_2=>false}
my_hash.is_a? Hash # or do whatever you want with your Hash
rescue JSON::ParserError
false
end
end
valid_hash? string1
#=> true
valid_hash? string2
#=> true
valid_hash? string3
#=> false